1. Field of the Invention
The present invention relates to the pulverizing of solid material and particularly to the production of a stream of finely pulverized coal entrained in a carrier gas. More specifically, this invention is directed to a grinding mill, wherein coarse material separated from a stream of pulverized material entrained in a carrier gas is mixed with incoming unpulverized raw material and the mixture subsequently delivered to grinding apparatus, and especially to a bowl mill which includes at least a first stirrer located immediately upstream, in the direction of incoming material flow, of the rotary bowl. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
Rolling mills, and particularly bowl mills, are well known in the art and, for example, are employed in the production of finely pulverized coal which is subsequently entrained in a carrier gas and used for charging steam generators of the type employed by electrical utilities. In a conventional rolling mill type pulverizer as employed by an electrical utility, "raw" coal is fed into a revolving bowl where size reduction takes place by virtue of the coal being crushed by rotating rolls. The partially pulverized coal passes over the edge of the bowl and is entrained in a stream of gas. This gas stream may, for example, comprise hot air which causes drying of the partially pulverized coal. The entrained particulate material is carried upwardly into a separator located above the bowl. The separator rejects the larger particles which are returned to the bowl for further grinding. The fine particles which are not separated and returned to the grinding zone will pass from the mill and typically be delivered to the furnace for combustion.
A rolling mill of the type generally described above is shown, for example, in brochure 3000.KO 4/88 entitled "Bowl Mills" published by EVT Energie and Verfahrenstechnik GmbH of Stuttgart, Federal Republic of Germany. The mills shown in the aforementioned brochure are mainly used for grinding hard coal to produce coal dust for use in the charging of steam generators.
In the prior art, excessive vibration has often been encountered during the operation of bowl mills, particularly during grinding to a high level of fineness. It has been discovered that the vibration, and a related decrease in mill efficiency, results from a poor mixing of the feed material, the raw coal for example, with the partially pulverized material returned to the bowl from the separator. As a consequence of this poor mixing, the frictional forces between the mill rollers and the material being milled, or between the mill bowl and the material being milled, or within the material being milled itself, are not constant and the power required to drive the mill thus fluctuates. In addition, it is known that the power consumption of a pressure-stressed grinding material layer, as is present in a bowl mill, is dependent upon grain formation and the moisture content of the material being ground. Thus, in summary, the best grinding performance is achieved with a homogeneously mixed grinding material having the proper moisture content.